The present invention is generally drawn to a fuel cell interconnect and more particularly to a multi-layered ceramic interconnect possessing superior design and performance characteristics, as well as a method for making the same.
Demand for electrical generating equipment has increased substantially in recent years. As a result, numerous technologies have been developed to provide electricity, including traditional grid-based systems and more localized, distributed generation systems. Moreover, as demand continues to increase, many anticipate that the demand for distributed generation systems will also increase.
In response to this distributed generation need, particular attention has been paid to fuel cell systems. Fuel cells are electrochemical devices that convert the energy of a chemical reaction directly into electrical energy. The basic physical structure of a single fuel cell includes electrodes (an anode and a cathode) with an electrolyte located there between in contact with the electrodes. To produce electrochemical reactions at the electrodes, a fuel stream and an oxidant stream are supplied to the anode and cathode, respectively. The fuel cell electrochemically converts a portion of the chemical energy of the fuel in the fuel stream to electricity, while the remaining amount of the chemical energy is released as heat. A stack of individual fuel cells is preferably connected in electrical series via a series of conductive interconnects in order to generate a useful additive voltage.
The type of electrolyte used in a fuel cell is generally employed to classify the fuel cell and is also determinative of certain fuel cell operating characteristics, such as operating temperature. Present classes of fuel cells include the Polymer Electrolyte Fuel Cell (PEFC), the Alkaline Fuel Cell (AFC), the Phosphoric Acid Fuel Cell (PAFC), the Molten Carbonate Fuel Cell (MCFC), and the Solid Oxide Fuel Cell (SOFC). In turn, interconnect devices are selected based upon compatibility with the electrolyte.
Current research efforts have focused upon SOFC systems for a variety of safety and performance concerns. For example, in comparison to other types of fuel cells, SOFC systems do not require the presence of corrosive electrolytes. Moreover, SOFC operating temperatures are more efficient and more conducive to integration with internal system components and/or other applications. Finally, given the ability of an SOFC stack to have either a tubular or planar shape, SOFC stacks afford greater design flexibility.
Planar solid oxide fuel cells are an attractive option for meeting the growing need for distributed power generation in a manner which is both energy efficient and environmentally sound. In particular, such systems offer modularity as well as higher fuel efficiency, lower emissions, and less noise and vibration in comparison to other distributed generation systems (e.g., diesel generators, gas turbines, etc.). However, the construction and operating costs of solid oxide fuel cells must compare favorably with these alternative power sources. Thus, in order to be widely accepted for distributed generation applications, solid oxide fuel cells must be able to cost-effectively produce electricity, as well as efficiently utilize the heat energy required to operate the cell.
Ideally, fuel cell performance should depend only on the fuel composition and the amount of fuel consumed at the anode side. Consequently, proper distribution of the reactant gases to the various parts of the cell is of particular concern. Various designs of anode-electrolyte-cathode tri-layers and associated flow passages are available for constructing fuel cell stacks. The most common configuration is the planar design having multiple layers of cell units stacked therein. The fuel and oxidant (e.g., air) respectively flow past the surface of the anode and cathode placed opposite the electrolyte. This arrangement allows the anode surface to be in direct contact with the fuel, and the cathode surface to be in direct contact with air. The flow passages for each gas can be connected to inlet and outlet manifolds on both the anode and cathode sides. Additional external baffles may also be provided to help channel the flow of reactant gases.
Generally speaking, the fuel is consumed due to electrochemical reactions as it passes across the anode from the inlet to the outlet. One function of the interconnect in a fuel cell stack is to insure distribution of fuel to all active areas of the cell. During cell operation, fuel must be supplied to even the most fuel-starved portions of the cell in a quantity sufficient to insure the proper operation of that fuel-starved portion. As a result, excess fuel is ultimately supplied to the entire cell in order to meet the demands imposed by the fuel-starved portions of the cell. This excess fuel usage has a negative impact on the overall cell and stack efficiency. Consequently, stack performance can be enhanced by improving the flow distribution of reactant gases within the cell.
Notwithstanding the issues associated with the negative impact of improper reactant flow on performance, SOFC interconnect functionality and interconnect cost actually constitute the greatest barriers to producing market competitive SOFC systems at present. In contrast to the flow patterns discussed above, the interconnect must provide reactant gas separation and containment, mechanical support to the cells, and a low resistance path for electrical current. Moreover, the reactant gas flow channels associated with the interconnect must be designed to control distribution of reactants with minimal pressure drop in the overall SOFC stack, especially in respect to the air flow channels of the interconnect because of the relatively high air flow rates required to dispose of heat from the stack. Finally, when integrated into the stack, each interconnect must be resistant to deleterious reactions (such as corrosion), dense to provide adequate gas separation of the reactant gases and still strong enough to minimize the effects of displacement cause by differential thermal expansion.
Monolithic interconnects made of lanthanum chromite ceramics and high-temperature metallic alloys have been used to address these problems with some amount of success. However, both types of interconnects are expensive and compromise aspects of the interconnect function. Moreover, lanthanum chromite and high-temperature alloys (for example, high chrome alloys) used in a conventional monolithic interconnect design are currently cost-prohibitive, although use of lanthanum chromite interconnect could theoretically allow for a marginally competitive product, assuming a regular, high production volume was needed and net-shape ceramic processing was used. In any event, lanthanum chromite provides a specific illustration of the basic conundrum in SOFC commercialization—the chilling effects of production start-up costs coupled with the initially small market size.
The gas separation requirement presents another problem in terms of materials selection. Obviously, the interconnect must present a barrier to separate the various gases flowing therethrough. Thus, a dense impermeable material with high electronic conductivity but almost no ionic conductivity must be used. Although ceramic processing has developed the capability to produce interconnects of sufficiently high density, many ceramics, including lanthanum chromite, have an unacceptably high ionic conductivity (thereby resulting in poor system performance). Many electrically conductive ceramic materials also exhibit undesired dimension changes when subjected to reducing gas atmospheres due to the loss of oxygen ions within the material. Alternate compositions of ceramic materials possessing low ionic conductivity generally have less than acceptable electronic conductivity or have a coefficient of thermal expansion (CTE) that is not well matched to that of the cell.
In contrast, metallic alloy interconnects have been developed that readily satisfy the gas separation function, but they generally do not exhibit adequate resistance to corrosion (and other deleterious reactions). In particular, oxide scale growth/formation and unacceptably high electrical resistance are probably the most challenging hurdles presented by known metal interconnects. Scale resistance is a function of oxide conductivity, thickness and continuity. Porous or laminar scales have the effect of increasing the current path length while reducing the effective current carrying cross sectional area. The mechanism for scale growth and conductivity are interrelated such that growth rate generally increases with scale conductivity. Higher growth rates tend to produce less dense, poorly adherent scales. Most alloys (except noble or semi-noble metals) actually trade advantageous scale conductivity for increased degradation because of scale growth. Coating the interconnect with a conductive oxide layer provides more control of the scale composition and microstructure, but does not change the basic nature of the problem. The application of coatings to alloy interconnects also increases the fabrication cost.
Regardless of the choice of ceramic or metallic interconnect, a close match of the cell and interconnect coefficients of thermal expansion is an absolute requirement. A close match of the CTE allows for the effective sealing of individual cells to interconnects and the concurrent containment of the reactant gases therein. Too large of a mismatch of CTE results in certain regions of the cell becoming adversely displaced. This physical displacement prevents effective confinement of the reactant gases within their intended flow paths, thereby adversely effecting performance of the entire SOFC stack. While changes between room and operating temperatures (generally in the range of 700 to 1000° C.) produce the largest thermal displacements, smaller temperature gradients across the stack (which vary with stack operating conditions) can also create detrimental displacements.
Dissimilar thermal expansion characteristics may also cause disruption of the electrical current path between cells and interconnects in a stack because of the relative movement of the contact points. Essentially, this loss of contact creates additional, unwanted resistance which substantially degrades stack performance and efficiency.
Most alloy interconnects also have a higher CTE in comparison to the other cell components. As a result, metallic alloy interconnects are particularly susceptible to contact resistance problems because the relative motion caused by expansion can dislodge a protective oxide scale and expose underlying unprotected metal. In turn, oxidation of any unprotected surface increases the overall scale thickness, and as mentioned above, the scale conductivity is comparatively poor so that such scale growth contributes directly to performance degradation. Additionally, oxide scales can adhere to the electrodes adjacent the interconnect. In such cases, relative motion can actually crack or damage the electrodes or the electrolyte layer itself.
In contrast, lanthanum chromite does not experience the same problems as alloy interconnects. Generally, the CTE of chromite ceramic interconnects is more closely matched to the cell; however, other concerns make these interconnects less attractive.
U.S. Pat. No. 6,183,897 to Hartvigsen et al., and assigned to SOFCo, a wholly owned subsidiary of McDermott Technology Inc., attempts to address some of the problems above. Its entire disclosure is incorporated by reference herein.
Hartvigsen proposes a ceramic SOFC interconnect with electrically conductive filled vias penetrating a gas separator plate. While Hartvigsen's design provides the resulting cell/stack with the gas separation qualities (by way of the separator plate) and excellent current collection and conduction (by way of the filled vias), Hartvigsen fails to discuss any means for optimizing the reactant flowfields, nor does it imply these items could be integrated into the interconnect itself. Likewise, Hartvigsen does not consider the complexities involved in providing a thermally compliant interconnect structure (e.g., column 6, lines 1-12).
U.S. Pat. No. 6,376,117 entitled “Internal Fuel Staging For Improved Fuel Cell Performance,” contemplates the inclusion of staging plates within the fuel cell stack to enhance reactant gas distribution along the tri-layer. However, the staging plates of this pending application must be provided separately from the interconnect itself, and the application fails to consider any sort of integrated structure. Notably, at present, this application is assigned to the same inventive entity as the present invention, and its entire disclosure is incorporated by reference herein.
Given the above, an interconnect which is well-matched to the components of an SOFC stack would be welcome. In particular, an interconnect which provides adequate flowpaths for the reactant gases and permits selective control of the performance of the cells/stack is especially needed.